The present invention relates to a system for controlling the air-fuel ratio for an internal combustion engine emission control system having a three-way catalyst, and more particularly to a system for controlling the air-fuel ratio so as to effectively operate the three-way catalyst.
Such a system is a feedback control system, in which an O.sub.2 -sensor is provided to sense the oxygen content of the exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of an air-fuel mixture supplied by a carburetor. The control system comprises a comparator for comparing the output signal of the O.sub.2 -sensor with a predetermined value, a proportional and integrating circuit connected to the comparator, a driving circuit for producing square wave pulses from the output signal of the proportional and integrating circuit, and an on-off type electromagnetic valves for correcting the air-fuel ratio of the mixture. The control system operates to determine whether the feedback signal form the O.sub.2 -sensor is higher or lower than the predetermined value corresponding to a stoichiometric air-fuel ratio for producing an error signal for actuating the one-off electromagnetic valve to thereby control the air-fuel ratio of the mixture.
In such a control system, if the engine is rapidly accelerated, the oxygen concentration of the exhaust gases is greatly deviated from a standard value corresponding to the stoichiometric air-fuel ratio. In order to cause the large deviation to quickly converge, the acceleration is detected and a shift signal depending on the detected acceleration is applied to the proportional and integrating circuit without waiting for the signal from the O.sub.2 -sensor corresponding to the large deviation. The shift signal causes an error signal to greatly shift, so that the air-fuel ratio of the mixture may be greatly shifted to control the oxygen concentration of the exhaust gases to the standard value. When the acceleration finishes, the shift signal disappears and the system returns to the feedback control operation. However, if the control operation for correcting the deviation of the concentration of the exhaust gases delays, the feedback control operation starts before the correction of the deviation. Therefore, the oxygen concentration of the exhaust gases is controlled at the deviated value. Consequently, the correction of the deviation of the oxygen concentration is delayed.